Mary ?, wife of John Agnew, d. abt 1780 b. Kilwaughter

I have found an entry in the Registry Books for a Mary who was born in Kilwaughter and married John Agnew.She died 1854 in Belfast at age 74.This would make her birth date about 1780.She is buried at the same place as Robert Agnew, engineer,who was buried in 1859 at age 50.His wife Elizabeth, a native of Belfast, was buried there in 1887 at age 74.Also buried in the same location is Mary Kelly, wife of John Kelly, who died in 1920 at age 61and is identified on the gravestone as Robert's daughter.Does anyone know who Mary Agnew was?Did John also come from Kilwaughter?